The present method of mixing asphalt and stabilizer has been basically the same for a number of years. This method utilizes a mixing chamber having multiple rotating paddles into which asphalt and filler are introduced in measured amounts. After thorough mixing, the composition is discharged from a single opening into a processor holding vessel. The measuring of the asphalt and stabilizer as it is fed into the mixer is controlled by volume. The asphalt is heated and fed to the mixer by a positive displacement gear pump. This pump has a controllable speed so as to vary its output as well as measure volumetrically through the tooth spaces and revolutions per minute of the pump. The stabilizer is generally introduced either through a helicoid screw turning at a designated rpm and delivering stabilizer in measured volumetric amounts or the system uses a rotary vane feeder that receives the filler in the voids between the vanes and measures it again volumetrically.
Asphalt weight by volume, varys depending on the temperature of the asphalt, therefore any variation in asphalt temperature varys the weight per cubic foot of the asphalt. The displacement pump, which is used to deliver the asphalt, also has variables in the degree of accuracy or efficiency of the pump thus producing variables in measuring the amount fed to the mixer. The accuracy of the pump is also affected by the pressure at which the asphalt is brought up to the pump. Any variation in pressure can change the measurement of the asphalt as it passes through the pump.
With respect to the stabilizer, there are a number of materials that can be used for this purpose. However, each stabilizer has a wide variation in weight by cubic measurement. Dolemite and limestone, depending on the grading and the region or country from which it is procured, can vary from 60 to 80 lbs. per cubic foot. Slate dust, oyster shells, available in the southern areas of the country, and fly ash are also used as stabilizers. Each of these also varies in weight, both with respect to each other and with respect to the other materials mentioned.
The equipment used for volumetric measurement of the stabilizer also has inherent variables. The wear factor on a rotary helicoid screw is very great and as the wear occurs the amount of material delivered per revolution changes. Vane type feeders are less susceptible to wear but also have this variable. The feed-in from the hopper to either the helicoid served or to the vane feeder can vary. There is no way of knowing whether the amount of material delivered to the screw or vane is consistent with what the screw or vane will take away.